A San Francisco judge on Monday upheld a jury verdict that found Monsanto liable for not warning a groundskeeper that its weed killer product Roundup might cause cancer, but slashed the damages award.

Judge Suzanne Bolanos denied Monsanto's request for a new trial but cut the $289 million damages award to $78 million to comply with the law regarding how punitive damages awards must be calculated.

Jurors in August unanimously found that Monsanto acted with "malice" and that its weed killers Roundup and the professional grade version RangerPro contributed "substantially" to Dewayne Johnson's terminal illness.

The jury ordered Monsanto to pay $250 million in punitive damages along with compensatory damages and other costs, bringing the total figure to nearly $290 million.

Johnson, a California groundskeeper diagnosed in 2014 with non-Hodgkin's lymphoma -- a cancer that affects white blood cells -- says he repeatedly used a professional form of Roundup while working at a school in Benicia, California.

"Although we believe a reduction in punitive damages was unwarranted and we are weighing the options, we are pleased the Court did not disturb the verdict," Johnson's lawyers said in a released statement.

"The evidence presented to this jury was, quite frankly, overwhelming."

In her ruling, Bolanos gave Johnson the choice of accepting the lessened damages award or triggering a new trial focused on what Monsanto should pay in the case.

Johnson's lawyers told AFP they are considering which option to pursue.

Monsanto attorney George Lombardi argued in court earlier this month that the evidence presented at trial did not support the jury verdict, and that an attorney for Johnson was wrong to urge jurors to teach the company a lesson.

In motions filed after the August verdict, Monsanto urged the judge to strip away the entire $250 million punitive portion of the damages, arguing that a new trial was justified.

Johnson's lawsuit built on 2015 findings by the International Agency for Research on Cancer, part of the UN World Health Organization, which classified Roundup's main ingredient glyphosate as a probable carcinogen, causing the state of California to follow suit.

Monsanto vice president Scott Partridge told reporters outside the courthouse the day of the verdict that "the jury got it wrong."

Monsanto has defended the weed killer, arguing that it has a 40-year history of safe use.